We are engaged in a phylogenetic survey of invertebrate muscle to determine the distribution, content relative to myosin, subcellular location and degree of evolutionary conservatism of the muscle protein, paramyosin. We have demonstrated a band on SDS gels of all the invertebrate muscles we have studied so far with identical electrophoretic mobility to that of Limulus paramyosin. Quantitative analysis of a statistical sampling of gels has provided molecular ratios of paramyosin to myosin. Fluorescence immunohistochemistry localizes paramyosin to the A-bands in muscles with a sarcomeric organization, in addition to confirming the protein's presence and demonstrating its immunologic similarity across phylogenetic lines. Our results, using other immunologic techniques, confirm the similarity among paramyosins. We are now extending these studies to muscles in other phyla, to paramyosin localization at the electron microscope level, and to structural analysis of thick filaments reconstituted using varying relative amounts of pure Limulus paramyosin and myosin. Using our data and the morphological data provided by our collaborators, Dr. Maynard M. Dewey and Dr. Benjamin Walcott, SUNY at Stony Brook, N.Y., we hope to establish a role for paramyosin related to both thick filament organization and the functional properties characteristic of those muscles which contain this protein.